A process for the laser welding of a tube with a probe that can be inserted in the tube is known, for example, from EP-A1-0 300 458. The probe described is connected by means of an optical waveguide with an Nd:YAG solid-state laser. The laser beam that emerges from one end of the optical waveguide within the probe is focused on a focal point located outside the tube by means of a lens system consisting of several lenses and a deflecting mirror. The deflecting mirror is inclined at an angle of 45.degree. to the longitudinal axis of the probe and deflects by 90.degree. the laser beam that is focused by the lens system and extends within the probe between the lens system and the deflecting mirror. The deflected laser beam leaves the probe through a cylindrical outlet opening located radially in the housing of the probe.
The probe is also provided with a flow channel for a shield gas, which runs through the wall of the housing of the probe, opens into the outlet opening, and flows out into the space between the deflecting mirror and the welding area.
In this familiar device, the protective gas stream is thus split directly opposite the welding area into a partial gas stream directed into the interior of the probe and a partial gas stream directed toward the welding point. This leads to the creation of turbulence in the vicinity of the outlet opening, so that welding vapor, welding plasma, or--particularly when a pulsed laser is employed--drops released from the melt may rebound on the deflecting mirror and at the outlet opening, thus considerably reducing the useful life of the probe.
One purpose of the present invention is therefore to provide a device and a process for the laser welding of a tube along its inner surface with a probe that can be inserted in the tube, with which any precipitation of welding vapor on the deflecting mirror and in the vicinity of the outlet opening is reduced to a large extent.